1. Field of the Invention
The present invention relates to a technology for positioning various types of functional components and devices between side plates in a frame.
2. Description of the Related Art
An image forming apparatus is one of devices that incorporate various functional components and devices in a frame structure including side plates disposed oppositely.
A full-color printer or the like that can form an image of not only a single color but a plurality of colors is widely used recently.
In the full-color printer, two systems are known. One of them is such that color images formed in a plurality of imaging units are sequentially transferred to a recording medium such as a recording sheet. The other one is such that, as a primary transfer process, images of different colors are sequentially transferred to an intermediate transfer member to be superposed and, as a secondary transfer process, the superposed images are collectively transferred to a recording medium that is conveyed to the intermediate transfer member. Reference may be had to, for example, Japanese Patent Application Laid-Open No. 2004-302194.
When images of different colors are transferred to be superposed on each other, it is necessary to prevent occurrence of any abnormal image due to color shift by improving transfer positional accuracy of color images.
One of factors affecting on the transfer positional accuracy is accuracy as to whether components of imaging units used to form images are accurately fixed in place. More specifically, if image carriers are not arranged in parallel to each other between the imaging units, or if light-path lengths of scanning lights are different from one another in a main scanning direction, then transfer positions deviate from one another between the imaging units. The deviation causes a transfer start position of an image to be different from the other ones when images are superposed, which may cause color shift to occur.
For example, Japanese Patent Application Laid-Open Nos. 2003-270898, and 2002-169353 disclose a structure to prevent degradation of fixing accuracy of devices between imaging units is such that a stay is provided laterally between side plates of a frame structure which is used to support these devices. Japanese Patent Application Laid-Open No. 2004-302194 discloses another structure such that a positional relationship between side plates and a stay is determined by using a jig and then the devices are fixed in place.
Japanese Utility Model Application Laid-Open No. H04-85445 discloses a structure to fix the stay laterally provided between the side plates is such that a plurality of convex portions is provided in the stay side and a plurality of concave portions is provided in the side plates at positions corresponding to the convex portions so that the convex portions can be fitted in the concave portions. One of the convex portions and the corresponding one of the concave portions are set as reference, and errors in the positions and the sizes of the other convex portions and concave portions are suppressed to values in their tolerance ranges respectively.
In a structure to prevent a tilt of side plates easily occurring when an external force affects the side plates with the stay arranged therebetween as described in Japanese Patent Application Laid-Open Nos. 2003-270898 and 2002-169353, the rigidity of the side plates can be enhanced by reinforcing the side plates using the stay. To enhance the rigidity of the side plates, the rigidity against the external force in particular, the rigidity of the side plates themselves are required. Hence, a sheet metal material, such as SECC (Steel, Electrogalvanized, ColdRolled, Coil) that is comparatively easy to ensure the rigidity, is often used for the side plates.
When the side plates are made of the sheet metal material, however, the weight of the frame structure increases, which may cause the entire image forming apparatus to increase its weight.
For this reason, a resin molding may be used for support structural components such as side plates to reduce the weight of the frame structure and the cost thereof.
When the resin molding is used, however, it is difficult for the resin molding, which is smaller than the sheet metal material, to ensure the rigidity. Hence, it can be thought of that a sheet metal component is integrally attached to part of the resin molding to ensure the rigidity at the fixed position of each device while its weight is reduced.
When this structure is used, a mold becomes complicated because the mold needs to be made allowing for incorporating the sheet metal component therein, and it is therefore difficult to obtain a complicated shape. In addition to this, when the sheet metal material is fixed to the part thereof, using the sheet metal material causes the increase of the material cost and the assembly cost as compared with the case where only the resin molding is used.
At the same time, the device is quite often fixed between the side plates by fastening. Therefore, positional accuracy between the devices cannot sometimes be ensured caused by displacement due to the fastening upon the fixing of the devices thereto although the rigidity of the side plates is ensured. This problem may also arise when supporting portions are formed by the resin molding. Even if the resin molding allows reduction of the weight of the device, upsizing of the components, and reduction of additional components due to integral molding, the displacement of the fixed position of the device as the resin molding may cause positions of images superposed on each other to be displaced.
When the structure disclosed in Japanese Utility Model Application Laid-Open No. H04-85445 is used, in particular, deformation of the stay occurring due to the external force affected after the stay is fixed can not be prevented even if the displacement occurring upon fixing of the device can be resolved.
If the stay provided between the side plates is distorted due to displacement at a fastened portion upon fixing of the device or due to an external force affecting the device after the device is fixed, the external force is easily concentrated on the fastened portion between the device and the side plates. This concentration may cause displacement of the fixed device.
Particularly, as described in Japanese Patent Application Laid-Open Nos. 2003-270898 and 2002-169353, when the stay is integrated to the side plates by fastening, the positional relationship between the side plates is restricted by the positional relationship between the side plates and the stay, and the positional relationship between the side plates and a device fixed thereto is thereby restricted. Because of the restriction, the fixed position of the device may remain displaced as it is. Besides, as described in Japanese Patent Application Laid-Open No. 2004-302194, when the side plates are to be positioned by using the jig, the stay, of which positional relationship with the side plates is determined by the jig, is distorted due to the external force after the jig is removed, which may cause displacement between the side plates.